1. Field of the invention
The subject matter of the present invention is a replant knee joint prosthesis assembly replacing a previously implanted prosthesis.
2. Prior Art
The replacement prosthesis must satisfy increased constraints, since the knee of the patient no longer has ligaments.
A replacement prosthesis must, despite the absence of ligaments, and most of the time due to the advanced age of the patient, substantially permit the latter to ensure a safe working of the articulation system, whatever the complexity of the movement of this articulation.
In fact it is a compromise between the muscular means of the patient that are at the disposal of the surgeon and the higher possibility of movement that a safe technology can offer.
In this manner appeared the replacement prostheses which are still little used at the present time, but which tend to be more and more used due to the quick increase of the lifetime of patients.
A knee joint prosthesis has to attempt to reproduce the different rotations of the natural articulation. However, a replacement prosthesis cannot perform these to the same extent and the constraints that are attributed to it are somewhat less important.
Generally speaking, it can be said that a good replacement prosthesis should satisfy qualities well higher than the hinged articulated prosthesis, i.e. a prosthesis the articulation of which is mechanically constrained by a connection axis and so can move only in one plane, the sagittal plane. Additionally, this good replacement prosthesis must allow the closest possible movement from the prosthesis, the so called posterostabilized movements, that tend to reproduce the natural knee articulation, without however allowing the smallest dislocation or release of the mechanical members between them.
The knee prosthesis with a patella joint articulation satisfies the criteria of a replacement prosthesis; this comes From the fact that the patella is a sphere which is necessarily located at the center of rotation and is used as a central support of the articulation, without for this reason constraining the latter during its movements. This is true if the latter brings both condyles to a friction identical or quasi identical on corresponding portions of the tibial disc or plate, or if one of the condyles is more strongly than the other put into contact with its corresponding portion of the tibial disc.
However, this kind of articulation does not exclude to respect the articulation interline of both knees of the patient, which, to allow the patient a good reproduction of working, requires that the articulations of each knee, and consequently their respective contact lines, lie in the same horizomtal plane, It must then be possible to technically act to this end.
Today, a few patents disclose a knee joint prosthesis with a patella articulation.
For example, U.S. Pat. No. 3,868,730 teaches a prosthesis of which the tibial shank, the tibial base plate slidingly receiving the condyles, and the articulation sphere, are made in one piece, thus making impossible any adjustment of the articulation interline and increasing subsequent wear and tear on the tibial disc, Moreover, the presence of the patella or sphere, in extension from the tibial shank, constitutes, in the working space between the femoral distal area and the tibial proximal area, a trouble for the working of the surgeon during the mounting of the femoral element.
Also the German patent DE 4 102 509 discloses a prosthesis with a patella joint articulation in which the catch and head assembly, which is removable, is introduced into an accommodation bore performed on the tibial system and has a length sufficient for efficiently resisting the miscellaneous mechanical constraints exerted on the patella joint head Due to the offset between the catch axis and the axis of the tibial system, the latter must be very cumbersome. This requires an intramedullar canal of corresponding size, and therefore entails a partial destruction of the bone capital, which considerably renders the canal brittle.
In this prosthesis, as in the British Patent GB-A-2 088 724, the joint is essentially realized between the condyles and the tibial discs. However, there is still wear and tear on the tibial disc.
At last, the European Patent EP-A-0 639 358 also discloses a ball and socket knee joint prosthesis in which the element forming the catch is in one piece with the tibial shank and the disc supporting plate. This assembly is integral as is the assembly of U.S. Pat. No. 3,868,730, but the ball head, instead of being part of this block, is interchangeable. As in the above U.S. patent, the catch of the prosthesis disclosed by this European patent must necessarily be attached to the stem and plate member (tibial base), as all mechanical efforts developed on the articulation are added to the same amplified efforts of the spherical head, itself mounted on the head portion of the catch. If the bearing surface of the head on the catch is small, this arrangement is not satisfying.
In this European patent, for adjusting the location of the articular interline in the horizontal plane formed with the plane of the other knee, it is sufficient to have tibial plates of different thicknesses and ball heads whose machining of the fitting diameter on the catch end is sufficiently reduced for allowing a total or partial fitting, the effect of which is to slightly raise the articulation.
Finally, the solutions proposed according to the prior art are poorly adapted, expensive and have a low safety. Indeed, it is never possible to modify the catch height since the latter is stationary and attached to the prosthesis which, as has become a rule for orthopedic surgeons, must respect as well as possible the bone capital of the patient.
The present invention specifically concerns a ball and socket joint replacement prosthesis, in which the catch supporting the spherical head is always offset from the stem or shank axis, which corresponds to the axis of the tibial intramedullar canal.
Sliding prostheses, which are so far the most comfortable, comprise a tibial implant which rolls and slides on the femoral implant. These prostheses are very close to a natural joint.
Sliding prostheses present nevertheless a disadvantage, i.e., a possible displacement translating itself by an anterior sliding and particularly a lateral one. This inconvenience appears notably when a replacement prosthesis is implanted when replacing a previous prosthesis.
On the other hand, sliding prostheses exhibit a dilapidation of the tibial part due to the wear and tear on the disc, which generally is formed from polyethylene, by chafing on the femoral implant.
The major inconvenience of these prostheses comes from the loss of freedom of movement of one piece with respect to the other, limiting the joint to function only in one plane, the sagittal plane
To address these problems there has been developed a sliding knee prosthesis equipped with a centering stud which avoids the sliding phenomena, for example the prosthesis described in above commented GB-A-2 088 724 and DE-A-4 102 509.
The present invention aims to remedy these diverse disadvantages while proposing a highly durable spherical knee joint prosthesis, prohibiting all transitory movement of one piece with respect to the other while allowing their rotation.